In a liquid crystal display device, a classification based on the operating mode for liquid crystals includes a phase change (PC) mode, a twisted nematic (TN) mode, a super twisted nematic (STN) mode, an electrically controlled birefringence (ECB) mode, an optically compensated bend (OCB) mode, an in-plane switching (IPS) mode, a vertical alignment (VA) mode, a fringe field switching (FFS) mode, a polymer sustained alignment (PSA) mode and a field induced photo-reactive alignment (FPA) mode. A classification based on the driving mode in the device includes passive matrix (PM) and active matrix (AM) types. The PM type is classified into static and multiplex types, etc. The AM type is classified into thin film transistor (TFT) and metal insulator metal (MIM) types, etc. The TFT type is further classified into amorphous silicon and polycrystal silicon types, wherein the latter is classified into a high temperature type and a low temperature type according to the production process. A classification based on the light source includes a reflective type utilizing natural light, a transmissive type utilizing backlight and a transflective type utilizing both the natural light and the backlight.
The devices include a liquid crystal composition having suitable characteristics. The liquid crystal composition has a nematic phase. General characteristics of the composition should be improved to obtain an AM device having good general characteristics. Table 1 below summarizes the relationship between the general characteristics of the two aspects. The general characteristics of the composition will be further explained based on a commercially available AM device. The temperature range of the nematic phase relates to the temperature range in which the device can be used. The preferred maximum temperature of the nematic phase is about 70° C. or higher and the preferred minimum temperature of the same is about −20° C. or lower. The viscosity of the composition relates to the response time in the device. A short response time is preferred for displaying moving images on the device. Accordingly, a small viscosity in the composition is preferred. A small viscosity at a low temperature is further preferred.
TABLE 1General Characteristics of Composition and AM DeviceGeneral Characteristics ofGeneral Characteristics ofNo.CompositionAM Device1Wide temperature range of aWide usable temperature rangenematic phase2Small viscosity 1)Short response time3Suitable optical anisotropyLarge contrast ratio4Large positive or negativeLow threshold voltage,dielectric anisotropySmall electric power consumption,Large contrast ratio5Large specific resistanceLarge voltage holding ratio,large contrast ratio6High stability toLong service lifeultraviolet light and heat7Large elastic constantShort response time,large contrast ratio1) A liquid crystal composition can be injected into a liquid crystal cell in a shorter period of time.
An optical anisotropy of the composition relates to a contrast ratio in the device. A product (Δn×d) of the optical anisotropy (Δn) of the composition and a cell gap (d) in the device is designed so as to maximize the contrast ratio. A suitable value of the product depends on a type of the operating mode. The suitable value is in the range of about 0.30 micrometer to about 0.40 micrometer in a device having the VA mode, and in the range of about 0.20 micrometer to about 0.30 micrometer in a device having the IPS mode or the FFS mode. In the above case, a composition having a large optical anisotropy is preferred for a device having a small cell gap. A large absolute value of dielectric anisotropy in the composition contributes to a low threshold voltage, a small electric power consumption and a large contrast ratio in the device. Accordingly, the large absolute value of the dielectric anisotropy is preferred. A large specific resistance in the composition contributes to a large voltage holding ratio and a large contrast ratio in the device. Accordingly, a composition having a large specific resistance at room temperature and also at a high temperature in an initial stage is preferred. A composition having a large specific resistance at room temperature and also at a high temperature even after the device has been used for a long period of time is preferred. Stability of the composition to ultraviolet light and heat relates to a service life of the liquid crystal display device. In the case where the stability is high, the device has a long service life. Such characteristics are preferred for an AM device for use in a liquid crystal projector, a liquid crystal television and so forth. A large elastic constant in the composition contributes to a short response time and a large contrast ratio in the device. Therefore, a large elastic constant is preferred. A small ratio of viscosity to an elastic constant in the composition contributes to a short response time in the device. Therefore, the small ratio of the viscosity to the elastic constants is preferred.
A composition having a positive dielectric anisotropy is used for an AM device having the TN mode. On the other hand, a composition having a negative dielectric anisotropy is used for an AM device having the VA mode. A composition having a positive or negative dielectric anisotropy is used for an AM device having the IPS mode or the FFS mode. A composition having a positive or negative dielectric anisotropy is used for an AM device having the PSA mode or the FPA mode. Examples of the liquid crystal composition are disclosed in Patent literature No. 1 to Patent literature No. 3.